Superheated steam water treatment process

ABSTRACT

Methods and apparatus produce steam and, more particularly, utilize untreated feedwater as a source for steam used in steam assisted gravity drainage. Superheated steam, produced from treated feedwater in a boiler, is used to vaporize untreated feedwater that would otherwise foul a boiler. Contaminants in the untreated water can them be removed as solids or concentrated brine. The vaporization process occurs in stages to allow for the use of a higher fraction of untreated water.

PRIORITY CLAIM

This application is a non-provisional application which claims benefitunder 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/731,242filed Nov. 29, 2012, entitled “Superheated Steam Water TreatmentProcess,” which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not Applicable

FIELD OF THE INVENTION

The invention relates generally to a method and apparatus of producingsteam and, more particularly, to a method utilizing untreated feedwateras a source for steam used in enhanced oil recovery. Superheated steamfrom treated water is contacted with untreated feedwater in multiplesequential stages to allow for a higher fraction of untreated water tobe vaporized.

BACKGROUND OF THE INVENTION

Conventional oil reserves are preferred sources of oil because theyprovide a high ratio of extracted energy over energy used for theextraction and refining processes it undergoes. Unfortunately, due tothe physics of fluid flow, not all conventional oil can be produced.Additionally, as conventional oil sources become scarce or economicallynon-viable due to depletion, unconventional oil sources are beingexplored as a potential supply of oil. However, unconventional oilproduction is also problematic because it consists of extra heavy oilshaving a consistency ranging from that of heavy molasses to a solid atroom temperature and heavy oils may also be located in reservoir rocks.These properties make it difficult to simply pump the unconventional oilout of the ground. Thus, its production is a less efficient process thanconvention oil.

As a result, enhanced oil recovery (EOR) techniques are often employedto increase the amount of heavy crude oil extracted. Using EOR, 30-60%or more of the original oil in place can be extracted. Additionally, EORtechniques can be applied in both conventional and unconventional oilreserves.

During EOR, compounds not naturally found in the reservoir are injectedinto the reservoir to assist in oil recovery. Simply stated, EORtechniques overcome the physical forces holding the oil hydrocarbonsunderground. There are many types of EOR techniques that are categorizedby the injection: gas injection, chemical injection, microbial injectionor thermal recovery. While there are many types of EOR techniques,reservoirs containing heavier crude oils tend to be more amenable tothermal EOR methods, which heat the crude oil to reduce its viscosityand thus decrease the mobility ratio. The increased heat reduces thesurface tension of the oil and increases the mobility of the oil.

A summary of various EOR techniques is presented in Table 1.

TABLE 1 Enhanced Oil Recovery (EOR) Techniques CSS Cyclic SteamStimulation or “huff and puff.” Steam is injected into a well at atemperature of 300-340° C. for a period of weeks to months. The well isallowed to sit for days to weeks to allow heat to soak into theformation, and, later, the hot oil is pumped out of the well for weeksor months. Once the production rate falls off, the well is put throughanother cycle of steam injection, soak and production. This process isrepeated until the cost of injecting steam becomes higher than the moneymade from producing oil. Recovery factors are around 20 to 25%, but thecost to inject steam is high. SAGD Steam Assisted Gravity Drainage usesat least two horizontal wells--one at the bottom of the formation andanother about 5 meters above it. Steam is injected into the upper well,the heat melts the heavy oil, which allows it to drain by gravity intothe lower well, where it is pumped to the surface. SAGD is cheaper thanCSS, allows very high oil production rates, and recovers up to 60% ofthe oil in place. VAPEX Vapor Extraction Process is similar to SAGD, butinstead of steam, hydrocarbon solvents are injected into an upper wellto dilute heavy oil and enables the diluted heavy oil to flow into alower well. ISC In situ combustion involves a burning of a small amountof the oil in situ, the heat thereby mobilizing the heavy oil. THAI Toeto Heel Air Injection is an ISC method that combines a vertical airinjection well with a horizontal production well. The process ignitesoil in the reservoir and creates a vertical wall of fire moving from the“toe” of the horizontal well toward the “heel”, which burns the heavieroil components and upgrades some of the heavy bitumen into lighter oilright in the formation. COGD Combustion Overhead Gravity Drainage isanother ISC method that employs a number of vertical air injection wellsabove a horizontal production well located at the base of the bitumenpay zone. An initial Steam Cycle similar to CSS is used to prepare thebitumen for ignition and mobility. Following that cycle, air is injectedinto the vertical wells, igniting the upper bitumen and mobilizing(through heating) the lower bitumen to flow into the production well. Itis expected that COGD will result in water savings of 80% compared toSAGD. EM A variety of electromagnetic methods of heating oil in situ arealso being developed. GAS A variety of gas injection methods are alsoused or being developed, including INJECTION the use of cryogenic gases.COMBO Any of the above methods can be used in combination.

While many EOR techniques involve injecting steam into undergroundformations, SAGD is the most favored form of EOR involving steam. It isespecially useful for the recovery of semi-solid crude oil known asbitumen. In SAGD, steam is injected into an upper horizontal injectionwell, which creates a steam chamber, and mobilizes the oil at the edgesof the chamber. The live oil then gravity drains to a lower horizontalproduction well, where the oil and water mixture is then collected.Large amounts of steam are needed for this operation, and in SAGD thesteam to oil ratio (SOR) is typically about 3, and can easily go higher.

Currently, most steam generators used for EOR are small, portable,once-through type units fired with oil or gas. The most common type ofSAGD boiler is the Once Through Steam Generator (OTSG), which generatessteam through indirect heat transfer. As evidenced by the name,water/liquid enters the system and makes a single-pass by the heatexchanger, vaporizes as it travels and exits as a steam/vapor mix. Themain advantage of OTSGs is its lower capital cost and ability to handlewater with relatively high percentage of dissolved and suspended solids,and organic contaminants.

In SAGD, high-pressure saturated steam is produced in boilers anddelivered to the wellpad, where it is injected into the SAGD reservoirs.However, the necessary water to oil ratio is very high. For every barrelof bitumen recovered, 2 to 4 barrels of water are needed.

Moreover, the steam condenses during contact and is coproduced with oil.Thus, both water separation and subsequent water treatment are necessaryoperations in heavy oil recovery. Because vast amounts of water areneeded to generate the required steam, a method of recycling theproduced water is essential for a cost effective, sustainable SAGDsystem. Furthermore, concerns about climate change have encouraged thedevelopment of ‘zero-emissions power generation.’

Produced water and brackish well water are the main boiler feedwatersources used for SAGD. But, both sources of water contain contaminants,particularly dissolved solids, which cause scaling or fouling of boilersystems. Fouling or scale from the contaminants can result in failure ofboiler tubes, down time to blow-down of the boiler and/or loss of boilerefficiency.

Normally, an OTSG can produce about 75-80% quality steam from feedwaterwith total dissolved solid (TDS) levels of 3,000 to 8,000 ppm. Thisrelatively low steam quality is necessary to maintain wet conditions inthe OTSG tubes in order to reduce fouling and scaling, but results inhigh blow-down levels of 20-25%. Although OTSG feedwater has relativelyhigh TDS levels, it still requires some treatment to reduce silica andhardness levels. This is typically accomplished by warm lime softeningfollowed by ion exchange. This water treatment process represents asignificant portion of surface facility capital costs, and has asignificant economic impact on a SAGD operation. Thus, what is needed inthe art is a method of recycling untreated water for steam generationwithout pretreatment, yet without fouling the boiler systems.

U.S. Pat. No. 4,398,603 describes a method of using low qualityfeedwater to produce steam. Here, feedwater is recycled and contactedwith superheated steam to produce saturated steam and precipitatedminerals. The precipitated minerals are removed by withdrawing a streamof waste water containing the minerals from the contacting vessel.However, this method requires a steam compressor that is notcommercially available.

Other steam generating methods also result in large amounts of CO₂ beingformed and subsequently co-injected with the steam. US20120160187discloses the use of an oxygen-fuel combustor as a steam generator,instead of a more traditional boiler system. This new steam generationsystem is able to use untreated water to produce 100% quality steam forEOR techniques. By using an oxygen-fuel steam generator, no chemicalsare needed to treat the water, regardless of total solid content becausethe heating is direct, rather than indirect heating as in a boiler. Theoxygen-fuel generator produces less than 100% quality steam and a brinecontaining contaminates. The brine is then removed via a steamseparator, resulting in 100% quality steam. However, this process alsogenerates CO₂ that is used during the injection process.

Betzer-Zilevitch et al. (2010) disclose another “Direct Contact SteamGeneration” system in which untreated water is heated by direct contactwith combustion gases, as opposed to the non-direct heating seen inOTSGs. However, the resulting steam is again mixed with a highpercentage of CO₂, which is then co-injected into the well.

The presence of CO₂ in the steam injection can be problematic forparticular types of underground formations, and the combination of CO₂and water can produce a corrosive mixture of carbonic acids that attackthe carbon steel typically used in injection pipes.

US20110061610 discloses a method of using water from waste streams.Here, the untreated water is preheated in a heat exchanger beforeentering a dryer, wherein input steam is used to indirectly dry(evaporate) the heated untreated water. This method reduces the amountof energy need to dry the untreated water while still producinghigh-quality water. The resulting steam is recycled in the dryer.However, during the drying process, the contaminants form a solid cakethat, upon further processing, can be used to backfill the reservoir.

Thus, what is needed in the art is a method for generating steam thatlowers water treatment costs and still avoids boiler fouling and theresultant costs, preferably a method that requires no pretreatment ofuntreated water is needed. Preferably, this method will also utilizecurrent steam generator and steam/water separation methods withoutexpensive modifications.

SUMMARY OF THE INVENTION

Embodiments of the invention describe a method of utilizing superheatedsteam to vaporize untreated water for use in enhanced oil recoverytechniques, preferably SAGD. The vaporization occurs in stages, thusallowing for a greater fraction of untreated water to be utilized. Indoing so, the water treatment cost of SAGD surface facilities aredecreased.

Some embodiments meet one or more of the following objectives.

A general objective is the design of an apparatus and method forgenerating steam that is simple in design, economic to build, maintainand operate, and is sufficiently rugged for wellpad use.

Another objective is the design of an apparatus and method forgenerating steam from untreated water to reduce water treatment cost.

Another objective is the ability to reduce boiler fouling and anyresulting boiler blow-down time.

Another objective is the adaptability of the present invention to steamgenerating systems currently in use with little modification.

In one embodiment, superheated steam, generated by a boiler or afurnace, is directly contacted with untreated water to vaporize some orall of the untreated water. The contaminants in the untreated water areremoved as solids if all of the water is vaporized. Otherwise, thecontaminants can be removed as a concentrated brine if only partialvaporization occurs. Both can be removed simultaneously in a suitablesteam/water separator (such as a cyclonic separator) or solid and liquidseparators can be used sequentially.

This results in a larger amount of steam that is significantly cooledwith respect to the original superheated steam. The steam isre-superheated in an indirect heat furnace, and then directly contactedwith more untreated water in a second stage. Again, the contaminants areremoved. The process is repeated for multiple stages. At each stage,successively larger amounts of untreated water are contacted withsuperheated steam.

The initial superheated steam is heated to about 900-1000° F. beforemixing with the initial untreated feedwater. The boiler or furnace usedto generate superheated steam can be any commercial available unitcapable of superheating steam.

Later stages of superheated steam (initial steam+steam from untreatedwater) are reheated to 900-1000° F. via a furnace. The superheated steamcan either by produced in superheater coils placed in the radiantsection of a boiler (common practice in power generation boilers), or ina stand-alone fired steam superheater.

In another embodiment, the mixing of steam and untreated water resultsin a wet steam plus liquid. Contaminants are then removed as aconcentrated brine. This concentrated brine, removed at each step, isvaporized in a single mixer and solids are removed in a single filter.The use of a single mixer and solid filtration device can lower overallcosts.

The contaminants are removed using well-known methods. In particular,for solids removal devices, cyclones and/or filters can be used. For aconcentrated brine, liquid/gas separation devices such as gravityseparators, centrifugal separators, and filter vane separators can beused.

In one aspect of the invention, untreated water with high levels oftotal dissolved solids can be used without any pretreatment step.

The term “boiler,” as used herein, denotes any means of indirectlyproducing superheated steam from feedwater before the initial contact ofsuperheated steam and untreated water, wherein the heat source is water.

The term “furnace” as used herein implies indirect heating of steam toincrease its level of superheat; wherein the heat source is ahydrocarbon such as gas or oil.

The term “untreated water” encompasses all water used for SAGD that hasnot undergone significant pretreatment to e.g., remove dissolved solidsbefore being heated and includes sources such as feedwater, brackishwater and water recovered from a production fluid.

The term “separators,” as used herein, mean any type of separationdevice used to separate components in different phases, i.e.solids/liquids, or liquids/gases.

The term “filter” refers to a device that separates solids from liquids(or solids) on the basis of particle retention and thus is size based.

The terms “mixer” and “contacting vessel” are used interchangeable andrefer to the vessel wherein the untreated water and superheated steamare contacted.

As used herein, the term “superheated steam” means a water vapor that is100% vaporized and at a temperature higher than its boiling point or atleast 482° C.

As used herein, “steam” refers generally to water vapor although theremay be some amounts of liquid water, water mist and solids therein.

“Saturated steam” is steam at the temperature of the boiling point whichcorresponds to its pressure; the term is sometimes also applied to wetsteam, and the terms are used interchangeably herein. “Slightlysaturated steam” is steam at a temperature 2.5-16° C. higher than itsboiling point.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims or the specification means one or more thanone, unless the context dictates otherwise.

The term “about” means the stated value plus or minus the margin oferror of measurement or plus or minus 10% if no method of measurement isindicated.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or if thealternatives are mutually exclusive.

The terms “comprise”, “have”, “include” and “contain” (and theirvariants) are open-ended linking verbs and allow the addition of otherelements when used in a claim.

The phrase “consisting of” is closed, and excludes all additionalelements.

The phrase “consisting essentially of” excludes additional materialelements, but allows the inclusions of non-material elements that do notsubstantially change the nature of the invention.

The following abbreviations are used herein:

BPD Barrels per day OTSG Once Through Steam Generator SAGD SteamAssisted Gravity Drainage SEP Separation device TDS Total dissolvedsolids

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Block flow diagram of process that uses superheated steam forthe staged vaporization of untreated water wherein solids are removedafter each vaporization step.

FIG. 2. Block flow diagram of process that uses superheated steam forthe staged vaporization of untreated water wherein concentrated brine isremoved after each vaporization step.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention provide a novel method of producing steamto be used in enhanced oil recovery techniques. In general, steamproduced from a treated water source is superheated, and the superheatedsteam then used in one or more stages to directly vaporize untreatedwater. The resulting steam is easily separated from any solidcontaminants using well-known solid filtration devices.

The method thus uses multiple stages for superheated steam/untreatedwater contact. Essentially, the initial vaporized untreated water andsteam are superheated after the solids are removed and then directed toa second stage to mix with more untreated water. Again, the resultingvaporized untreated water is superheated and contacted with moreuntreated water in a third stage. In some embodiments, this processrepeats multiple times, for a minimum of 3 stages, preferably a minimumof 5 stages. By using a staged vaporization, a higher fraction ofuntreated water can be converted to steam, thereby reducing watertreatment cost associated with SAGD surface facilities.

In more detail, a treated water source is superheated to about 482-538°C. using a boiler or fired steam superheater. This initial superheatedsteam is then mixed with an untreated feedwater stream in a 2.5 to 4.5ratio in a contactor vessel. This mixing results in a less heated steamand solid minerals or concentrated brine. The brine and solid mineralsare removed from the less heated steam using a solid/liquid separatingdevice or a liquid/gas separating device. The less heated steam is thenre-superheated to about 482-538° C. using a furnace. This larger volumeof steam is then mixed with a new amount of untreated feedwater inanother contacting vessel. The process repeats at least two times,resulting in larger quantities of untreated feedwater being convertedinto less heated steam. After the final mixing, the less heated steam isinjected in a well for mobilizing heavy oil.

The present invention is exemplified with respect to FIGS. 1 and 2.However, these figures are exemplary only, and the invention can bebroadly applied to any steam generating system and any source ofuntreated water. The following examples are intended to be illustrativeonly, and not unduly limit the scope of the appended claims.

Solid Precipitation

Exemplary results, generated by process modeling, of the basic steamgeneration system depicted in FIG. 1 for reducing the amount of treatedfeedwater is given in Table 2. Referring to FIG. 1, superheated steam ismixed with untreated water in five separate contacting vessels.Different contacting vessels have to be used at each stage becauseprogressively lower operating pressures are necessary. In the initialmixing, superheated steam is mixed with untreated water in a 3.3 massratio. During mixing, the water is converted into a less heated steam,resulting in a final composition of steam and solid contaminants. Thesolids are filtered out using a solids separation device such as acyclone separator and/or filter, and the steam flows into a furnace toreheat. The re-superheated steam is directed into a second contactingvessel with a new batch of untreated water and the mixing/separationprocess is repeated.

In this particular example, 20,000 barrel per day (bpd) of treatedfeedwater was converted into 538° C. superheated steam in a conventionalboiler. That superheated steam is mixed with 6,000 bpd of untreatedwater in the first contacting vessel (‘mixer’). The resulting mixture is26,000 bpd of slightly saturated steam that is about 316° C. and solidcontaminants. As the slightly saturated steam is directed to thefurnace, it passes a solids separation device (‘filter’). Once thesolids are removed, the slightly saturated steam is re-superheated inthe furnace to about 538° C. This superheated steam is directed into asecond mixer with 7,900 bpd of fresh untreated water. The processrepeats, with increasing amounts of untreated water being utilized ateach stage.

Table 2 displays the fraction of treated and untreated water as afunction of mixing stages. As shown, increasing the number of stagesdecreases the amount of treated feedwater needed. As such, moreuntreated water is utilized, thus reducing traditional water treatmentcost of the facility.

TABLE 2 Relative quantities of treated and untreated feedwater as afunction of number of vaporization stages. Number of VaporizationTreated Water (% of Untreated Water (% of Stages total feedwater) totalfeedwater) 1 77% 23% 2 59% 41% 3 45% 55% 4 35% 65% 5 27% 73%

Concentrated Brine Removal

FIG. 2 depicts a steam generating system wherein a concentrated brine isfiltered out, as opposed to actual solids. As in FIG. 1, the superheatedsteam is mixed with untreated water in a contacting vessel in fiveseparate stages. During mixing, the untreated water is transformed intoa less heated steam, resulting in a final composition of steam andconcentrated brine contaminants.

The concentrated brine is separated out using a gas/liquid separator(‘Sep’). The brine can then be vaporized in a single mixer with thesolids being removed via a single filter afterwards. Note, this differsfrom FIG. 1, in that only one solid removal device is needed for allfive stages. A single filter reduces cost and system complexity.

After the brine is separated out, the remaining steam is directed into afurnace to be reheated. A second stream of superheated steam is added tothe wet steam before it enters the furnace. This second stream is addedto vaporize any droplets carried over from the mixers, which preventsthe droplets from drying and fouling the furnace tubes. After beingreheated, the superheated steam is streamed into a second contactingvessel with a new batch of untreated water and the mixing/separationprocess is repeated.

Concentrated Brine and Solids Removal

Some embodiments allow for the removal of both solids and concentratedbrine. This design is similar to FIG. 2 except a solid/gas filter islocated in-line after the gas-liquid separator in the ‘Sep’.Alternatively, the solid/gas filter could also be located in-line withthe gas-liquid separator, but after the makeup steam line. Thisconfiguration would allow one system to separate out either solids orconcentrated brine, depending on the needs of the technique.

The following references are incorporated by reference in theirentirety.

Betzer-Zilvitch, M. “Integrated Steam Generation Process and System forEnhanced Oil Recovery,” Conference Paper, Society of PetroleumEngineers, GSUG/SPE 137633, 2010.

-   U.S. Pat. No. 4,398,603-   US20120160187-   US20110061610

What is claimed is:
 1. A method of producing steam for heavy oilrecovery, comprising: a) introducing an initial superheated steam into acontactor vessel; b) introducing a feedwater stream into said contactorvessel; c) mixing said initial superheated steam and said feedwater toproduce a less heated steam and contaminants in said contactor vessel;d) separating said less heated steam from said contaminants; e) againsuperheating said less heated steam in a furnace to create a superheatedsteam and repeating steps a-e in sequential contactor vessels ofdecreasing pressure; f) flowing a final less heated steam from a finalcontactor vessel; and g) injecting said final less heated steam into aninjection well for the mobilization of heavy oil.
 2. The method of claim1, wherein said contaminants are solid minerals.
 3. The method of claim1, wherein separation of the less heated steam from said contaminants ispreformed using a solids separation device.
 4. The method of claim 1,wherein separation of the less heated steam from said contaminants ispreformed using a cyclone or filter.
 5. The method of claim 1, whereinsaid contaminants are in a liquid phase and separated by a gas-liquidseparator.
 6. The method of claim 1, where said contaminants areseparated as concentrated brine that undergoes further vaporization in asingle mixer and solids are thereafter removed via a separator.
 7. Themethod of claim 1, where said contaminants are separated as concentratedbrine that undergoes further vaporization in a single mixer and solidsare thereafter removed via a cyclone or filter.
 8. The method of claim1, further comprising adding additional superheated steam to the lessheated steam, between steps d and e, after being separated from thecontaminants and before superheating again in the furnace.
 9. The methodof claim 1, wherein said initial superheated steam is heated to 482-538°C. using a power plant boiler or a fired steam superheater.
 10. Themethod of claim 1, wherein said initial superheated steam is heated to482-538° C. using a power plant boiler.
 11. The method of claim 1,wherein the initial ratio of said superheated steam to said feedwater is2.5-4.5.
 12. The method of claim 1, wherein the initial ratio of saidsuperheated steam to said feedwater is 3.3.
 13. The method of claim 1,wherein said superheated steam is 482-538° C.
 14. A steam productionsystem for heavy oil recovery, comprising: a) a boiler; b) a furnace; c)n mixers and n separators for separating steam from solid and/orliquids, wherein each of said n mixer is fluidly connected to an nseparator which is fluidly connected to said furnace, and wherein thefurnace is fluidly connected to each of said n mixer, and wherein n isat least 3; d) n inlet lines connected to each of said n mixers forfeeding untreated water to each mixer; e) said boiler fluidly connectedto a first one of the mixers; and f) a last one of the mixers fluidlyconnected to a heavy oil injection well.
 15. The system of claim 14,wherein the mixers contact the untreated water and superheated steam toproduce a less heated steam and contaminants removed by the separatorsin a plurality of stages and the furnace superheats said less heatedsteam received from said mixers and separators and thereby generatesadditional superheated steam directed to a next stage of the mixers andseparators.
 16. The system of claim 14, further comprising a vessel forvaporizing said liquids removed from the steam by the separators and asolids removal device for separating solid-waste from resultingadditional steam generated in the vessel.
 17. The system of claim 14,wherein the separators are gas-liquid separators for removing brine fromthe steam generated in the mixers.
 18. The system of claim 14, whereinthe separators are solids separation devices for removing solid mineralsfrom the steam generated in the mixers.
 19. The system of claim 14,further comprising a pipe to combine an additional amount of superheatedsteam with the steam at an outlet from said separators but before saidfurnace.
 20. A method of steam assisted gravity drainage (SAGD),comprising: injecting steam into a horizontal injection well andrecovering produced hydrocarbons from a lower horizontal productionwell; and preparing the steam for said injection wells by i)superheating steam, ii) mixing said superheated steam with untreatedwater to produce steam and solids or concentrated brine or both, iii)separating said solids or concentrated brine from said steam in stepii), iv) re-superheating said steam in a furnace, and v) repeating stepsii-iv) at least two more times.